Structures of three salts of phthalic acid; variation in crystal packing and geometry of the hydrogen phthalate ion.

Abstract

The hydrogen-bond patterns in hydrogen phthalates (HP) have been elucidated by the determination of the crystal structures of hydrogen phthalate salts of [Mg(H(2)O)(4)-(CH(3)OH)(2)](2+), 2-(2'-pyridylamine)pyridinium and diethylammonium. The stoichiometry of the latter salt corresponds to a hydrogen phthalate salt; however, it contains the phthalic acid in its three possible acid-base forms. The hydrogen phthalate ions in the three salts display the two common hydrogen-bonding motifs. One has a very short intramolecular O-H-O hydrogen bond (Intra-H), as seen in the magnesium salt [O...O 2.397 (2) A]. In the other two salts the hydrogen phthalate ions are connected by hydrogen bonds into infinite chains (Chain-H) with O...O distances of 2.460 (1) A in the diethylamine salt and 2.610 (1) A in the 2,2'-dipyridylamine salt. This difference in hydrogen-bond pattern was further elucidated by a search in the Cambridge Structural Database for other hydrogen phthalate salts. Intra-H possesses the shortest O...O distances and an almost planar hydrogen phthalate ion, whereas the hydrogen phthalate ions show significantly more variation in the Chain-H salts. The two hydrogen-bonding motifs are reflected in the molecular geometry of the hydrogen phthalate ion.